1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to multiuser detection with time diversity and space diversity, and more specifically, relate to a space-time multiuser detector for simultaneously removing multiple access interference (MAN) and multipath channel interference that occur in multiple access systems such as code division multiple access (CDMA) systems.
2. Description of the Related Art
Most of current mobile communication systems suffer performance degradation due to multipath channel interference. Such degradation can be resolved using transmit diversity. At an initial stage, multiple transmitting antennas are used to execute transmit diversity, or, both multiple transmitting antennas and multiple receiving antennas are used to efficiently enhance transmit diversity and receive diversity for space-time. Thus, it is possible to avoid attenuation caused by multipath channel interference.
Commercially available code division multiple access (CDMA) scheme in conformity to Interim Standard 95 (IS-95) has limitations in the enhancement of speech quality and capacity of subscribers as the number of users increases. To overcome such limitations, various algorithms have been proposed to suppress multiuser interference.
Multiuser detection algorithms can be categorized into a linear multiuser detection algorithm and a subtractive interference cancellation detection algorithm. The linear multiuser detection algorithm includes a decorrelator and a minimum mean-square error (MMSE) detection algorithm. The subtractive interference cancellation detection algorithm includes a successive interference cancellation (SIC) detector and a parallel interference cancellation (PIC) detector. Hereafter, the multiuser detection algorithms are explained in sequence.
The decorrelator is similar to a zero-forcing equalizer used for removing intersymbol interference. The decorrelator removes the interference signal by taking an inverse matrix of a matrix built with a correlation value of code sequences of each user. However, the decorrelator is disadvantageous in that the number of computations drastically increases as the number of users increases since the decorrelator needs to acquire the inverse matrix. In addition, the noise increment is disadvantageous as the decorrelator conducts multiplication of the inverse matrix, rather than suppressing multiple access interference.
The MMSE detector minimizes mean-square error between actual data and a soft output of a conventional detector. However, the MMSE detector, which is transformed from the correlation matrix, needs to estimate an amplitude of a received signal. Additionally, as the capacity of the MMSE detector depends on the power of an interference user, loss due to near-far problems may arise and the inverse matrix is similarly required for the decorrelator.
The subtractive interference cancellation detection algorithm suppresses the multiuser interference with respect to the received signal in a serial or parallel manner, unlike the linear detector. The SIC detector detects signals in the descending order from the highest user power received, newly generates MAI components using the detected signals, and removes the generated MAI components from the original received signal in sequence. The SIC detector features very simple hardware configuration, but lengthens the delay time as the number of users increases.
The PIC detector suppresses the MAI components in parallel, differently from the SIC detector. Hence, the delay time can be reduced but the complex hardware configuration is hard to realize.
Currently, space-time multiuser detection algorithms having multiple transmitting and receiving antennas have been proposed to simultaneously suppress multipath channel interference and multiuser interference. A representative example of the space-time multiuser detection algorithms may incorporate the decorrelation algorithm, which is one of the linear multiuser detection algorithms, and the MMSE detection algorithm, when the multiple transmitting and receiving antennas are employed.
FIG. 1 is a block diagram of a multiple transmitting and receiving antenna system adopting a conventional space-time code. In the multiple transmitting and receiving antenna system, a transmitting end includes a space-time transmit diversity (STTD) encoder 100 and a plurality of transmitting antennas 102 and 104. A receiving end includes a plurality of receiving antennas 110 and 112, channel estimators 114 and 116, and a STTD decoder 118.
Symbols x1 and x2, passing through processes for transmitting data, such as channel encoding and interleaving, are input to the STTD encoder 100. The STUD encoder 100 encodes the input symbols x1 and x2 according to the STUD encoding. The STTD encoding of the input symbols at the STTD encoder 100 is explained below. First, the input symbols x1 and x2 are encoded according to the STTD encoding operation and output as the encoded symbols (x1, x2) and (−x2*, x1*). For instance, (x1, x2) output from the STTD encoder 100 is fed to the first transmitting antenna 102, and (−x2*, x1*) is fed to the N-th (N=2) transmitting antenna 104. The symbols output from the transmitting antennas 102 and 104 are received at the receiving antennas 110 and 112. Specifically, the symbol output from the first transmitting antenna 102 is received by the first receiving antenna 110 through the N-th receiving antenna 112. The symbol output from the N-th transmitting antenna 104 is received by the first receiving antenna 110 through the N-th receiving antenna 112. In other words, the first receiving antenna 110 receives all the symbols (x1, x2), (−x2*, x1*) received from the first transmitting antenna 102 and the N-th transmitting antenna 104, and the N-th receiving antenna 112 receives all the symbols (x1, x2), (−x2*, x1*) received from the first transmitting antenna 102 and the N-th transmitting antenna 104.
The symbol arriving at the first receiving antenna 110 is provided to the channel estimator 114, and the symbol arriving at the N-th receiving antenna 112 is provided to the channel estimator 116. The channel estimators 114 and 116 estimate the channels through which the received symbols have been transmitted, from the respective antennas. The STTD decoder 118 performs the STTD decoding of the received symbols by applying the estimated channel characteristics that are provided from the channel estimators 114, and 116. That is, the STTD decoder 118 performs STTD decoding by applying the characteristics of corresponding channel to the symbols (x1, x2), (−x2*, x1*) arriving at the first receiving antenna 110, and applying the characteristics of corresponding channel to the symbols (x1, x2), (−x2*, x1*) arriving at the N-th receiving antenna 112 respectively. Because the symbols at the first and the second receiving antennas 110 and 112 can be compared with each other, more accurate decoding can be provided. The symbols output from the STTD decoder 118 are fed to the detector, and the detector detects the fed symbols. In case of a system that transmits and receives data via multiple transmitting and receiving antennas, the receiving antenna obtains the transmitted signal by combining signals transmitted from the transmitting antennas.
Meanwhile, it is assumed that a plurality of transmitting antennas transfers different signals and a receiving antenna receives a signal from one of the plurality of the transmitting antennas. In detail, the first receiving antenna should receive a signal only from the first transmitting antenna, and the N-th receiving antenna should receive a signal only from the N-th transmitting antenna. However, the first receiving antenna may receive signals from the second through N-th transmitting antennas. In this case, the signals from the second and N-th transmitting antennas act as noise or interference signals. As a result, a novel method is demanded to remove the MAI components from multiuser interference and suppress multipath channel interference at the same time.